


i'm sucking your last words from the back of your throat

by avosettas



Category: Undertale (Video Game)
Genre: Breeding, Crosstale Sans (Undertale), Ecto-Breasts (Undertale), Ecto-Penis (Undertale), Ecto-Vagina (Undertale), Established Relationship, Knotting, M/M, Mating Cycles/In Heat, Nesting, Nipple Play, Praise Kink, Sans/Crosstale Sans (Undertale), Vaginal Sex, my dumbass forgetting tags yet again.
Language: English
Status: Completed
Published: 2021-01-05
Updated: 2021-01-05
Packaged: 2021-03-15 16:07:54
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,509
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28566690
Author URL: https://archiveofourown.org/users/avosettas/pseuds/avosettas
Summary: “you’re in heat,” Sans sighs, rubbing circles into Cross’s back with his thumb.“I know,” Cross says miserably, even as he softens under Sans’s touch.
Relationships: Sans/Sans (Undertale)
Comments: 3
Kudos: 147





	i'm sucking your last words from the back of your throat

**Author's Note:**

  * For [snasational](https://archiveofourown.org/users/snasational/gifts).



> iiiiiiit's 3am!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! goodnight!!!

The sight of the bare mattress stops Sans in his tracks. He’s sure it was made this morning when he got out of the shower - Cross was too neurotic to even leave the comforter undone, like Sans preferred to. But as he steps into their bedroom now, the mattress is completely bare, crooked on its frame, and all of its contents are missing. 

Including Sans’s mate, though locating him turns out to be quite easy, once Sans begins looking. Hidden by the bed, the comforter has been crumpled messily on the floor, half-spread out, with pillows and smaller, folded blankets arranged on top of it in a circular fashion. Some of their other extra blankets are spread out on it too, like the messily knitted one Elderberry had given Cross when he’d moved out, and the soft fleece one Cross would throw around his shoulders on cold nights. Embedded between the pillows and blankets are bits of clothing. Mostly socks, from the looks of it.

In the center of the little nest sits Cross, face flushed, looking like he might cry from frustration. Sans picks his way over to him carefully. “you okay?” 

“You’re back,” Cross says, like he can’t believe it. He reaches for Sans, and Sans lets him, surprised at the sudden affection. It’s not that Cross _isn’t_ affectionate - it’s just that he’s usually more hesitant about it. Nevertheless, Sans leans down, welcoming his purring mate into his arms with, well, open arms. 

Cross is _hot_ against him, and if it weren’t for the badly made nest Sans would assume he had a fever. He tilts his head to look down at Cross; most of his face is a spotty shade of pale purple, and, oh, there’s the embarrassment. He’s tense in Sans’s arms, despite how much his body and soul are telling him _go to your mate_. 

“you’re in heat,” Sans sighs, rubbing circles into Cross’s back with his thumb. 

“I know,” Cross says miserably, even as he softens under Sans’s touch. 

“...so what’s wrong?” Sans presses. Cross has made a nest, big enough to fit both himself and Sans inside, and he has his mate there, holding him in his arms. 

“The nest is bad,” Cross replies, his voice muffled as he buries his face in Sans’s sweatshirt. “I can’t get the nest right, and I can’t take care of you if I can’t get the nest right.” 

“...do y’want me to help you fix it?” Sans asks after a moment. “‘cause i don’t mind. i gotta take care of you too, after all.” 

Cross looks conflicted. What Sans has offered to do conflicts with the instincts his heat has given him - he’s meant to craft a nest that satisfies his mate, that will let the two of them raise a family together. But Sans had worked late today, and by the time Cross had even begun working on the nest, his motor skills had become impacted by his heat. “...Okay,” he agrees softly. 

Sans has to pry Cross from his waist so he can take a good look at the nest. As he does, Cross grabs his hand, and Sans chuckles a bit. “heat makes you real affectionate,” he observes as he surveys the nest, making mental notes in his head as he goes. 

The comforter will need to be spread out flat, with another blanket layered on top of it, to make sure it’s soft enough. The pillows and other blankets will need to be rearranged, and the folded blankets will need to be spread out more. Sans considers all of this in addition to Cross’s current lack of coordination. 

“help me move everything but the comforter off the floor,” Sans directs. Cross moves slowly; he’s probably in pain, probably exhausted and overloaded, but Sans knows he won’t rest yet, even if he was told to. Sans wouldn’t, either; the one thing that a heat really did to you was make you want to care for your mate, and make them a suitable place to nest in. 

Eventually, with Sans’s directions and minimal help - he would have liked to help more, but Cross refused to let him - the nest is remade to something that fits Cross’s standards. Cross kicks up a low purr as he pulls Sans into it. Sans finds himself being pressed against the pillows as Cross cuddles up against him, nuzzling his skull into San’s shoulder. 

“good?” Sans asks, managing to free his arm from under Cross’s body to stroke the other’s skull. 

“Yeah,” Cross replies, voice slightly warbled by his continuous purring. “Except…” 

“what’s up?” 

“...Can we… try for a kid this time?” The last bit is quiet, but Sans still hears it. He blinks, staring at the ceiling. 

He _knows_ Cross likes kids - hell, anyone who’d been around his mate for more than five minutes knew Cross liked kids. He just didn’t really know Cross wanted kids of his own. Sans has never really thought about it - sure, he sort of parented Paps, but… that was different. Paps was already elementary school aged when Sans became his primary caretaker. 

This would be a baby, growing from a tiny little outgrowth on one of their own souls and feeding off their magic. Completely different. 

“...Sans?” Cross asks. His eyes are wide and hopeful, but he turns his head and mumbles something about just forgetting it as Sans looks at him. 

“...i’m not saying no, but, uh, how lucid are you right now?” Cross huffs a laugh, and rolls his eyes. 

“Except for the nest thing, almost perfectly,” he brags, though his voice softens as he continues, “I’ve kind of… always wanted kids, I’ve just been too afraid to bring it up. I didn’t know how you’d feel about it…” 

Sans is still considering what he knows about heats, and how he’s experienced his own. Cross is right - aside from a few strong instincts, unless you ignore the urges for too long, it isn’t as if you lose your mind while in heat. And he knows it’s only been a few hours since Cross’s heat started; his mate was perfectly fine when he left this morning. 

“okay,” Sans agrees softly, and Cross lights up. His teeth meet Sans’s in an long, excited kiss. 

“You’re gonna be a great dad,” Cross assures him firmly, only inches away from Sans’s face. Softer, he says, “Would you, um… would you do me the, um, honor of carrying our child?” 

Sans feels his face flush, but that question is so _Cross_ that he can’t help but just nod. It’s so formal, and clearly rehearsed, yet it has all of Cross’s charm in it. “yeah, it’d be a real… honor,” he states stupidly, mind actually blanking on a pun.

Cross’s eyes are bright, pure white in the deep black of his eye sockets, and Sans can’t believe how happy this one little thing has made him. “I’ll take such good care of you,” he promises, clambering on top of Sans and draping his entire body over his mate’s. 

Sans snorts. “you always do,” he says as Cross nuzzles his cheek. 

Cross is still purring softly, but he finally stops nuzzling to press Sans’s body further down into the nest. It brackets Sans in the corner of the room, their bed on one side on the nest, a wall on the other, and a wall behind him. Cross is the final wall, closing the nest off from the outside world as he dips his head down to nuzzle at Sans yet again. 

He wouldn’t _normally_ classify himself as impatient, but Cross’s promise and his heat have driven Sans to frustration, and after allowing his mate to continue for another moment, he reaches up and pulls Cross’s mouth down to his. Cross doesn’t seem to mind, if the way he pushes Sans even further into the soft rim of the nest is any indication. 

Even in heat, Cross is only barely willing to take the lead, clumsily slipping his tongue into Sans’s mouth. His hands stay rather politely on Sans’s shoulders, though they grip tightly enough that there might be bruises when all is said and done. 

“you waitin’ for a signed invitation or somethin’?” Sans mumbles against Cross’s teeth, laughing at Cross’s embarrassed, but nevertheless grumpy expression. “thought i already got your RSVP, babe.” 

Cross doesn’t respond aside from a roll of his hazy eyelights, though he pushes off of Sans and starts pulling at his own clothes. The attempt ends early; it seems to involve too much coordination for his shaking hands, and he whines. 

“c’mere,” Sans murmurs, reaching for him. Cross’s response is immediate, and Sans has to work around his besotted mate to remove their clothes. Cross attempts to help, mostly with Sans’s clothes, but mostly he simply shoves the discarded clothing into the sides of the nest, instead of actually doing anything helpful. 

Except Cross looks upset still, and Sans has to rack his brains for whatever meager information on heat remains there. It turns out to be a useless venture - Cross, voice tinged with his earlier whine, simply says, “I need to take care of _you_ ” and that explains it all. That stupid caretaking instinct. 

Sans grins and lays back. “‘m all yours.” 

Cross’s face softens a bit, and he once again wraps himself around Sans. Now, with no clothes between them, his body feels less fevered and more _burning_ , and Sans wonders how he’s standing it. His joints are flushed the same soft purple as his face, and magic swirls in his pelvic inlet. 

(Sans is _very_ tempted to reach out and run his hand through that shapeless purple mist, maybe graze his fingers over Cross’s sacrum in the meantime… but, no. Not today, not while Cross is in heat, beholden to his instinct to take care of Sans.) 

“You’re so pretty…” Cross sighs, settling himself between Sans’s legs. His fingers ghost over Sans’s femurs as he adds, much softer, “...I wonder how you’ll look when you’re carrying our child… Probably even prettier.” 

Sans shivers a bit. Cross isn’t usually a talker during the act; he has to wonder if it’s the heat addling his brain just a bit, or if he’s just that into this. 

He doesn’t get all that long to wonder, though - with little preamble, Cross has once again covered Sans’s body with his own, playing his fingers over Sans’s ribs as he nibbles at his collarbone and cervical vertebrae. It’s soft, barely-there contact, but it has Sans taking deep breaths to regain the air he doesn’t technically need as his own magic begins to swell to the surface. 

Cross, above him, hums in his work, shakily as his purr returns yet again. His fingers travel along the inside of Sans’s ribcage; at the same time, Sans’s magic finally comes to life enough to shape into its own formless cloud above his pelvic inlet. Cross scratches at the underside of his sternum once, making Sans shiver, before retracting his hands, and then sitting back on his knees again. 

His ecto-body has come into being, at some point when Sans wasn’t paying attention - it’s a lovely shade of violet, stretching from the bottom of his sternum and ribs to his knees. His cock is fully hard already, dripping precum. His knot hasn’t yet swelled, though, Sans is pleased to see. That probably would have been unbearable to try and work through, and they’re both suckers for foreplay. 

Cross pushes Sans’s legs up against his ribs, holding them there with firm hands, and the stretch is almost uncomfortable, but more uncomfortable is how, at this angle, Sans’s entire pelvis is now bared. Cross isn’t looking at Sans’s pelvis and the cloud of magic within it, though. He’s looking straight at Sans’s face as he softly rubs his thumb over the Sans’s leg. 

“Hold them here?” It’s a question, not a command - a lifetime of being a soldier has made Cross hate such things, and they both know Sans would be more likely to fool around if he was being ordered around. 

When Cross releases his hands, Sans’s own replace them, and his focus changes to Sans’s pelvis, bared to the world. But Cross _is_ his world right now, keeping Sans safe and hidden in their nest, helped by the bed and the walls of their bedroom. 

Sans jumps when Cross’s hand touches his pubic arch. It’s sensitive, though not quite in the same way ecto-flesh is; he’s sure he couldn’t orgasm from having only his bones touched. 

Though he might be able to from Cross’s voice, softer and less steady than usual: “Can you…” 

Sans only hums in response, letting everything fall into place. Cross likes tits, so he manifests his breasts along with his pussy. Cross’s purr kicks up again, and he carefully leans between Sans’s legs to nuzzle at his collarbone. “Thank you.” 

“‘course,” Sans tries to say, though it cuts off with a short gasp as Cross’s entire weight rests on hip. His tongue licks at the bits of Sans’s breasts that are exposed beneath his knees, occasionally grazing the soft false flesh with his sharp canines. 

“I don’t know if skeletons lactate,” Cross murmurs partially to himself as he pushes Sans’s legs down to bare his torso so he can reach his breasts. Sans’s sighs as his feet rest back on the bottom of the nest. “I think we do; I bet your breasts will get all nice and full… They might hurt, though.” 

Sans groans as Cross nips at one of his nipples, then licks over it. “I think they’d look nice, all big and full of milk, but I don’t like it when you’re in pain.” Cross pushes himself up and sits back again. His hands return, not quite as warm as his mouth, but warm enough as they knead Sans’s chest slowly, but firmly. “I suppose that we can cross that bridge when we come to it.” 

“cross,” Sans wheezes as the other pinches at his nipples, tweaking them hard. “ _cross_.”

“Hm?” The look on Cross’s face is stupidly innocent, and Sans doesn’t even know what he wants to do to wipe it off his face - maybe sit on it. He can’t believe Cross has the capacity to be a tease while he’s in heat. 

“stop teasing,” Sans huffs, raising his hips, but Cross hardly bats an eye as he uses one hand to push them back down. His face is still flushed, but beneath that layer of purple he’s stern, hand pressing gently into the fat of Sans’s stomach. 

“I told you,” Cross says earnestly, grasping Sans’s hips. “I’m going to take really good care of you.” 

Sans grunts wordlessly as Cross’s hands leave his hips, trusting him to be good and keep his body in the nest. Cross doesn’t push his legs back up again; rather, he leans back down over Sans’s chest, and his hands come to rest on the pudge just above Sans’s waist. Mentally, Sans prepares himself for more teasing, and also makes a note to himself to never summon his tits if he wants Cross to _get the fuck on with it_. 

“You’re so pretty,” Cross breathes, breath hot on Sans’s chest, and Sans wonders once again how the hell he hasn’t combusted from his heat. The thought is gone almost as soon as it came, because Cross is pressing his teeth to Sans’s sternum, gently running his fangs down the false flesh covering it. “So soft…” 

Sans inhales shakily, and barely manages to get that breath back out as Cross once again decides to move his hands to knead his breasts. Every squeeze goes straight to his core, and it feels like _he’s_ the one in heat - he finds himself wanting to just raise his hips high enough to hump Cross like some sort of animal. His belly is resting right on Sans’s waist, _so close_. 

“cross,” Sans says, though it comes out too much like a whimper for his liking. He does his best to raise his hips, managing to _barely_ grind his clit against Cross’s abs. It’s not enough, and, perhaps worse, Cross notices his attempts and _moves_. 

“You’re really desperate today,” he murmurs. On his knees between Sans’s own legs again, his thumb rubs at the soft ecto of Sans’s thigh. Then, he grabs Sans’s other thigh, and flips him over. 

Sans is unprepared, and lands more-or-less on his face, cushioned by the rim of the nest. One of Cross’s hands has moved beneath him, on his stomach. “Are you okay?” 

“i’d be better if you’d just fuck me,” Sans growls, probably in a way in mate considers nonthreatening. 

Sure enough, Cross laughs, and his hand retracts from Sans’s belly. “Soon,” he promises. The blanket beneath them moves slightly as Cross scoots forward, and then Sans finds himself leaning forward even further as Cross spreads his legs. He braces himself in the pillows and twists his head to see if Cross is _finally getting on with it_. 

Cross smiles at him, and places both hands on his thighs. One thumb rubs over Sans’s labia, and he moans shamelessly - it’s the most he’s gotten so far. “You’re really wet,” Cross tells him needlessly, because, uh, _duh_. Then, Cross continues, still rubbing just that one finger through his slick, “Does the idea of me… breeding you really get you that excited?” He sounds almost shy, but Sans knows it gets _both_ of them excited. 

“Does it?” Cross presses, thumb sliding down to Sans’s clit, and he jolts, then nods, because he doesn’t trust himself to answer verbally. “Good,” Cross hums, folding himself over Sans’s back. “Gonna take care of you… Fuck you real good…” Cross’s purr rumbles through Sans’s body, and he shifts a hand and finally inserts a finger. 

“Take care of you after that too,” Cross mumbles, shallowly thrusting for a moment before slipping another finger in beside it. Sans lets out a shaky breath as Cross presses them in as far as he can get them, and then spreads them. “You’ll be all nice and heavy with our child, and I’m sure you’ll be lazier than ever…” His chuckle is slightly strained as he keeps going, repeating the motion. “And just as handsome, if not more,” Cross adds, his other hand making its way back up to Sans’s chest.

His head is on Sans’s lower back now, a perfect position to feel Sans shudder and whine as he continually pumps his fingers. Then, with one final splaying motion, the fingers disappear. Sans makes a wordless sound of discontent, something between a whine and a groan. Cross strokes his clean hand over Sans’s back as his weight disappears as well. 

Then, Cross’s cock is pressing into him, and Sans keens. Cross is hot against him from his heat, and Sans can’t _think_. Cross grips his hips firmly, though not hard enough to bruise, and he pulls out and fucks into him smoothly in one motion. Sans moans as he finds his pace, eventually settling on fast and smooth, never hard enough to hurt. 

“Sans,” Cross says, once again, leaning his weight into him, and Sans lets out a warbling moan in response, the best he can do. He thinks Cross is going to continue, but he doesn’t; instead, after a few more thrusts, Cross presses into him firmly, his knot slipping into Sans’s pussy. 

He thrusts a few more times, shallow and fast, before nipping at Sans’s shoulder as he cums. Sans whines as Cross pants behind him, then yelps slightly as he’s suddenly moved, still attached to Cross via his knot. 

He finds himself in Cross’s lap, back to his mate’s ribs, as Cross drops his hand down to fiercely rub at Sans’s clit. Sans throws his head back, and Cross is looking down at him, loving determination in his eyes. 

Sans comes with a quiet moan, and Cross bucks into him, oversensitive, as Sans clenches around him. Despite that, however, he still rubs Sans through his orgasm, leaving both of them gasping by the time he finally moves his hand. 

Then, he drops his head to nuzzle at Sans’s skull, and puts both hands on his belly. The purr he releases is even more obvious from this position, rumbling from Cross’s ribcage through Sans’s softly rattling bones. His thumbs stroke Sans’s belly gently, up and down. “Told you,” he mumbles against Sans’s skull. “Told you I’d take real good care of you, before, during, _and_ after…” His heat-flush has dissipated somewhat, though an embarrassed one reappears now. “..you know.” 

Sans laughs. “‘m not quite convinced,” he says as Cross splays one hand on the pudge of his belly. “why don’tcha do it again?” With a weak waggle of his browbone, Sans issues a challenge he thinks he’ll probably regret when Cross’s heat is over. “gonna have to breed me again, maybe a few time… real thoroughly, y’know? _really_ get your point across.” 

Cross’s determined nod makes him think that whatever soreness he’ll end up with will be worth it, though.

**Author's Note:**

> twitter @avosettas


End file.
